1. Field of the Invention
The present invention relates to a variable directivity condenser microphone having two condenser-type microphone units disposed on opposite sides of a fixed electrode, more particularly to a variable directivity condenser microphone using a vacuum tube as an impedance converter.
2. Description of the Related Art
As a microphone whose directivity is variable, one having two condenser-type microphone units disposed on opposite sides of a fixed electrode is known.
In this case, the above-mentioned two microphone units each have cardioid directivity. The variable directivity is realized by adjusting a polarization voltage applied to each unit or by controlling a degree of addition of an audio output signal from each unit. This is disclosed in Japanese Patent Application Publication No. H7-143595, Japanese Patent Application Publication No. 2012-65147, etc.
On the other hand, since the condenser-type microphone unit has a very high output impedance, FET (field-effect transistor) or a vacuum tube is used as an impedance converter. When using the latter vacuum tube as the impedance converter, it is usually necessary to prepare a dedicated power supply.
In other words, in operation, the above-mentioned vacuum tube needs one that is commonly referred to as an A-power supply for heating a heater of the vacuum tube and one that is commonly referred to as a B-power supply for supplying a high voltage to a plate of the vacuum tube.
In general, the vacuum tube used as the impedance converter needs a DC power supply which provides 6.3 V as the above-mentioned A-power supply and one that provides around 120 V as the above-mentioned B-power supply. Therefore, an external power supply dedicated to the vacuum tube is used, since a phantom (Phantom) power supply which provides DC 48 V and is often employed when using the above-mentioned FET as the impedance converter cannot supply sufficient electric power to drive the above-mentioned vacuum tube.
FIG. 2 shows an example of a conventional circuit block diagram, in which the vacuum tube is used as the impedance converter, and it is arranged that drive power is supplied from an external power supply circuit to the above-mentioned vacuum tube etc.
In FIG. 2, the external power supply circuit 3 surrounded by broken lines is connected to a microphone body 1 through a connector indicated by reference sign 2. Further, terminals (1), (2), and (3) of the connector 2 are connected with a mixer amplifier (not shown) etc., via a balanced shielded cable, through which an audio signal is outputted.
That is to say, the terminal (1) of the connector 2 is a ground (GND) connector terminal of the microphone body 1. The terminal (2) is a hot (HOT) side connector terminal for the audio signal, and the terminal (3) is a cold (COLD) side connector terminal for the audio signal.
Further, reference sign MC indicates a condenser-type microphone unit in which, on opposite sides of a fixed electrode BP, a front diaphragm FD (also referred to as first diaphragm) and a rear diaphragm RD (also referred to as second diaphragm) are arranged in front of it and behind it respectively. That is to say, a first microphone unit is constituted by the fixed electrode BP and the front diaphragm FD and a second microphone unit is constituted by the fixed electrode BP and the rear diaphragm RD. Furthermore, each of the above-mentioned first and the second microphone units has the cardioid directivity.
The above-mentioned central fixed electrode BP is connected to a point of reference potential of the microphone body 1, i.e., a ground (GND) connector terminal (terminal (1)) of the connector 2 through a resistance element R1. Further, a constant polarization voltage (for example, DC 120 V) is applied to the front diaphragm FD (which constitutes the first microphone unit) from an external power supply circuit 3 through a terminal (5) of the connector 2. Furthermore, it is arranged that the rear diaphragm RD which constitutes the second microphone unit is supplied with a positive or negative variable DC voltage from an external power supply circuit 3 through a terminal (6) of the connector 2.
The above-mentioned external power supply circuit 3 is provided with direct-current power supplies E1 and E2 (positive or negative) whose ends are connected to the above-mentioned point of reference potential respectively. Two ends of a potentiometer VR are respectively connected to a positive terminal of the DC power supply E1 and a negative terminal of the DC power supply E2. Further, a slide terminal of the potentiometer VR is connected to the above-mentioned terminal (6) of the connector 2.
Both the above-mentioned direct-current power supplies E1 and E2 (positive or negative) are set as 120 V, so that a polarization voltage which is varied from +120 V to −120 V by operation of the slide terminal of the potentiometer VR can be applied to the rear diaphragm RD which constitutes the second microphone unit.
On the other hand, the fixed electrode BP which constitutes the above-mentioned microphone unit MC is connected to a grid of a vacuum tube (triode) Q1. Further, a voltage of 120 V which is supplied to the above-mentioned front diaphragm FD as a polarization voltage is applied to a plate of the vacuum tube Q1 through a plate resistor R2.
Furthermore, a cathode resistor R3 and a capacitor element C1 are connected in parallel between a cathode of the above-mentioned vacuum tube Q1 and the point of the reference potential so as to constitute a cathode bias circuit, thereby applying a negative bias to the grid of the vacuum tube Q1 through the above-mentioned resistance element R1.
One end of a primary winding of a transformer T1 is connected to the plate of the above-mentioned vacuum tube Q1 through a coupling capacitor C2. The other end of the primary winding is connected to the point of reference potential. Further, ends of a secondary winding of the above-mentioned transformer T1 are respectively connected to the terminals (2) and (3) of the connector 2. An audio signal is outputted in parallel from the microphone body 1 to the above-mentioned mixer amplifier etc. through the above-mentioned terminals (2) and (3).
It should be noted that the A-power supply (6.3 V) is arranged to be supplied to a heater H1 of the vacuum tube Q1 from the external power supply circuit 3 through the terminal (4) of the connector 2, as shown in FIG. 2.
In the arrangement shown in FIG. 2, as described above, a polarization voltage of 120 V is always applied to the front diaphragm FD which constitutes the first microphone unit. Further, by adjusting the potentiometer VR provided for the external power supply circuit 3, an arbitrary polarization voltage within a range of from +120 V to −120 V can be applied to the rear diaphragm RD which constitutes the second microphone unit.
Therefore, in the case where, for example, +120 V is applied to the rear diaphragm RD (which constitutes the second microphone unit) by adjusting the potentiometer VR provided for the external power supply circuit 3, it is possible to obtain a directivity in which an output from the second microphone unit is added in phase to an output from the first microphone unit, i.e., a non-directional property.
Further, a polarization voltage of 0 V applied to the rear diaphragm RD which constitutes the second microphone unit does not generate an output from the second microphone unit, whereby, an output only from the first microphone unit, i.e., a cardioid directivity, can be obtained.
Furthermore, in the case where the polarization voltage applied to the rear diaphragm RD which constitutes the second microphone unit is −120 V, it is possible to obtain a directivity in which the output of the second microphone unit is subtracted from the output of the first microphone unit, i.e., a bidirectional property.
Incidentally, according to the variable directivity condenser microphone shown in FIG. 2, the external power supply circuit 3 provides the above-mentioned vacuum tube Q1 with A-power (supply) and B-power (supply) through the connector 2 of the microphone body 1, and the polarization voltage is also supplied to each of the diaphragms FD and RD of the microphone unit MC. In addition, in the circuit arrangement shown in FIG. 2, it is arranged that the above-mentioned B-power (supply) applied to the plate of the vacuum tube Q1 is also applied to the front diaphragm FD using as the polarization voltage. However, it is necessary for the polarization voltage applied to the rear diaphragm RD to be supplied through one of the terminals provided for the above-mentioned connector 2, i.e. the terminal (6) in FIG. 2.
As such, according to this type of condenser microphone which uses the vacuum tube as an impedance conversion means, there are a lot of connection wirings between the connector 2 provided on the microphone body 1 side and the external power supply circuit 3, so that an increase in the number of terminals provided for the above-mentioned connector and an increase in the number of connection wirings connected with the external power supply circuit side may reduce reliability of operation.
Therefore, it is a technically important subject to attain a decrease in the number of terminals used in the above-mentioned connector and simplification of the connection wirings between the connector and the external power supply circuit side. The present invention mainly aims to solve the above-mentioned problems.